The purpose of this project was to elucidate the mechanisms underlying cellular remodeling in different cell types in response to different agents. The response of L-cells to cyclic AMP and changes of shape of Fc receptor bearing lymphocytes in response to immobolized antigen antibody complexes have been studied. Methods employed included time-lapse photography, electron microscopy, treatment with various drugs, and in the case of L-cells, electrophysiology. Reconstructions have been made from serial thin sections of large areas of L-cell surfaces after treatment of cells with cyclic AMP or prostaglandin E1. The roles of various cellular organelles in shape changes have been outlined and some suggestive evidence as to some control mechanisms has been obtained. There are differences between cell types in the detailed patterns of changes involved in the metamorphosis from one shape to another.